The UK’s Government Communications Headquarters (GCHQ), alleged by Edward Snowden to be undertaking mass surveillance of online activities, operates under the Intelligence Services Act 1994. GCHQ’s first statutory function is “to monitor or interfere with electromagnetic, acoustic and other emissions and any equipment producing such emissions and to obtain and provide information derived from or related to such emissions or equipment and from encrypted material”.  GCHQ is understood to be exclusively responsible for large-scale interception, although it undoubtedly shares information with a range of other intelligence, policing and tax authorities.
GCHQ’s Director must ensure “that there are arrangements for securing that no information is obtained by GCHQ except so far as necessary for the proper discharge of its functions and that no information is disclosed by it except so far as necessary for that purpose or for the purpose of any criminal proceedings”.  These functions can be exercised ‘in the interests of national security, the economic well-being of the UK,’ and ‘in support of the prevention or detection of serious crime’. 
 Intelligence Services Act 1994, s.3 (1)(a).
 Intelligence Services Act 1994, s.4 (2).
 Intelligence Services Act 1994, s.3 (2).